p-well Process :

The fabrication steps of p well process are same as that of an n-well
process except that instead of n-well a p-well is implanted . The process
steps involved in p-well process are shown in Figure below. The process
starts with the n type substrate.

Step 1 :
A thin layer of SiO2 is deposited which will serve as the pad
oxide.

Step 3 :
A plasma etching process is used to create trenches used for insulating the
devices.

Step 4 :
The trenches are filled with SiO2 which is called as the field
oxide.

Step 5 :
To provide flat surface chemical mechanical planerization is performed and
also sacrificial nitride and pad oxide is removed.

Step 6 :
The p-well mask is used to expose only the p-well areas, after this implant
and annealing sequence is applied to adjust the well doping. This is
followed by a second implant step to adjust the threshold voltage of the
NMOS transistor.

Step 7 :
Implant step is performed to adjust the threshold voltage of PMOS
transistor.

Step 8 :
A thin layer of gate oxide and polysilicon is chemically deposited and
patterned with the help of polysilicon mask.

Step 9 :
Ion implantation to dope the source and drain regions of the PMOS (p+) and NMOS (n+) transistors, this will also form n + polysilicon gate and p+ polysilicon gate for NMOS
and PMOS transistors respectively. Hence this process is called as self
aligned process.

Step 10 :
Then the oxide and nitride spacers are formed by chemical vapour
deposition.

Step 11 :
In this step contact or via holes are etched, metal is deposited and
patterned. After the deposition of last metal layer final passivation or
overglass is deposited for protection.

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